The present invention relates to a fuel injection system as defined in the preamble of claim 1, and a method as defined in the preamble of claim 13.
The fuel injection system comprises piezoelectric elements being considered in more detail. Piezoelectric elements can be used as actuators because, as is known, they possess the property of contracting or expanding as a function of a voltage applied thereto or occurring therein.
The practical implementation of actuators using piezoelectric elements proves to be advantageous in particular if the actuator in question must perform rapid and/or frequent movements.
The use of piezoelectric elements as actuators proves to be advantageous, inter alia, in fuel injection nozzles for internal combustion engines. Reference is made, for example, to EP 0 371 469 B1 and to EP 0 379 182 B1 regarding the usability of piezoelectric elements in fuel injection nozzles.
Piezoelectric elements are capacitive elements which, as already partially alluded to above, contract and expand in accordance with the particular charge state or the voltage occurring therein or applied thereto. In the example of a fuel injection nozzle, expansion and contraction of piezoelectric elements is used to control valves that manipulate the linear strokes of injection needles. The use of piezoelectric elements with double acting, double seat valves to control corresponding injection needles in a fuel injection system is system is shown in German patent applications DE 197 42 073 A1 and DE 197 29 844 A1, which are incorporated herein in their entirety.
Fuel injection systems using piezoelectric elements are characterized by the fact that, to a first approximation, piezoelectric elements exhibit a proportional relationship between applied voltage and the linear expansion. In a fuel injection nozzle, for example, implemented as a double acting, double seat valve to control the linear stroke of a needle for fuel injection into a cylinder of an internal combustion engine, the amount of fuel injected into a corresponding cylinder is a function of the time the valve is open, and in the case of the use of a piezoelectric element, the activation voltage applied to the piezoelectric element.
FIG. 8 is a schematic representation of a fuel injection system using a piezoelectric element 2010 as an actuator. Referring to FIG. 8, the piezoelectric element 2010 is electrically energized to expand and contract in response to a given activation voltage. The piezoelectric element 2010 is coupled to a piston 2015. In the expanded state, the piezoelectric element 2010 causes the piston 2015 to protrude into a hydraulic adapter 2020 which contains a hydraulic fluid, for example fuel. As a result of the piezoelectric element""s expansion, a double acting control valve 2025 is hydraulically pushed away from hydraulic adapter 2020 and the valve plug 2035 is extended away from a first closed position 2040. The combination of double acting control valve 2025 and hollow bore 2050 is often referred to as double acting, double seat valve for the reason that when piezoelectric element 2010 is in an unexcited state, the double acting control valve 2025 rests in its first closed position 2040. On the other hand, when the piezoelectric element 2010 is fully extended, it rests in its second closed position 2030. The later position of valve plug 2035 is schematically represented with ghost lines in FIG. 8.
The fuel injection system comprises an injection needle 2070 allowing for injection of fuel from a pressurized fuel supply line 2060 into the cylinder (not shown). When the piezoelectric element 2010 is unexcited or when it is fully extended, the double acting control valve 2025 rests respectively in its first closed position 2040 or in its second closed position 2030. In either case, the hydraulic rail pressure maintains injection needle 2070 at a closed position. Thus, the fuel mixture does not enter into the cylinder (not shown). Conversely, when the piezoelectric element 2010 is excited such that double acting control valve 2025 is in the so-called mid-position with respect to the hollow bore 2050, then there is a pressure drop in the pressurized fuel supply line 2060. This pressure drop results in a pressure differential in the pressurized fuel supply line 2060 between the top and the bottom of the injection needle 2070 so that the injection needle 2070 is lifted allowing for fuel injection into the cylinder (not shown).
It is an object of the present invention to identify defective piezoelectric elements.
This object is achieved, according to the present invention, by way of the features claimed in claim 1 and in the characterizing portion of claim 13. An inventive fuel injection system with a piezoelectric element for controlling the amount of injected fuel, comprises a control unit for determination of a possible fault of the piezoelectric element or of an electric circuitry driving the piezoelectric element based upon a value related to the capacitance of the piezoelectric element. This provides for a very effective diagnosis of the piezoelectric element and/or the circuitry driving the piezoelectric element. The circuitry driving the piezoelectric element may comprise switches for selection of particular piezoelectric elements or the power stage. The present invention is preferably used for detecting faults, in particular shortcuts in switches for selecting particular piezoelectric elements. Especially if there is a defective cylinder selecting switch the corresponding piezoelectric element would charge every time in parallel to another piezoelectric element so that a cylinder, corresponding to that defective cylinder selecting switch is filled with such a large amount of fuel that serious damage to the engine could occur. If there is a shortcut in the connection between power stage and the piezoelectric element similar serious problems might occur.
In a preferred embodiment of the invention the fuel injection system comprises at least two piezoelectric elements for controlling the amount of injected fuel, wherein the control unit is able to determine a possible fault of at least one of the piezoelectric elements or the electric circuitry driving the piezoelectric elements based upon on values related to the capacitances of the at least two piezoelectric elements.
In a further preferred embodiment of the invention the piezoelectric element is charged from a first voltage to a second voltage, wherein the control unit determines the value related to the capacitance of the piezoelectric element based upon the first voltage and the second voltage.
In a further preferred embodiment of the invention the piezoelectric element is charged from a first voltage to a second voltage with in a charging time, wherein the control unit determines the value related to the capacitance of the piezoelectric element based upon the charging time, and, in particular an estimated value of, a current charging the piezoelectric element. In a further embodiment of the invention the value related to the capacitance of the piezoelectric element is the difference between the first voltage and the second voltage or a function of that difference if charging time and charging current are kept essentially constant (with respect to the time instances the values related to the capacitance is calculated for or with respect to different piezoelectric elements). In a further embodiment of the invention the value related to the capacitance of the piezoelectric element is the charging time or a function of the charging time, if the difference between the first voltage and the second voltage and the current charging the piezoelectric element are kept essentially constant (with respect to the time instances the values related to the capacitance is calculated for or with respect to different piezoelectric elements). In a further embodiment of the invention the value related to the capacitance of the piezoelectric element is the current charging the piezoelectric element or a function of the current if the difference between the first voltage and the second voltage and the charging time are kept essentially constant (with respect to the time instances the values related to the capacitance is calculated for or with respect to different piezoelectric elements).
In a further preferred embodiment of the invention the control unit determines the value related to the capacitance of the piezoelectric element based upon the charge the piezoelectric element is carrying. In a possible embodiment of the invention the value related to the capacitance of the piezoelectric element is the charge the piezoelectric element is carrying or a function of this charge if the (first, second, third) voltage is kept essentially constant (with respect to the time instances the values related to the capacitance is calculated for or with respect to different piezoelectric elements).
In a further preferred embodiment of the invention the control unit determines the value related to the capacitance of the piezoelectric element based upon the quotient of the second voltage and the charge the piezoelectric element is carrying, based upon the quotient of the charge the piezoelectric element is carrying and the second voltage, based upon the quotient of the difference between the second voltage and the first voltage and the product of the charging time and the current charging the piezoelectric element, or based upon the quotient of the product of the charging time and the current charging the piezoelectric element and the difference between the second voltage and the first voltage. If one of the quantities charging time, current charging the piezoelectric element or difference between second voltage and the first voltage is kept essentially constant (with respect to the time instances the values related to the capacitance is calculated for or with respect to different piezoelectric elements), it can be replaced by one or another constant value.
In a further preferred embodiment of the invention the piezoelectric element is discharged from a second voltage to a third voltage, wherein the control unit determines the value related to the capacitance of the piezoelectric element based upon the second voltage and the third voltage.
In a further preferred embodiment of the invention the piezoelectric element is discharged from a second voltage to a third voltage within a discharging time, wherein within a discharging time the control unit determines the value related to the capacitance of the piezoelectric element based upon the discharging time, and, in particular an estimated value of, a current discharging the piezoelectric element. In a further embodiment of the invention the value related to the capacitance of the piezoelectric element is the difference between the third voltage and the second voltage or a function of that difference if charging time and charging current are kept essentially constant (with respect to the time instances the values related to the capacitance is calculated for or with respect to different piezoelectric elements). In a further embodiment of the invention the value related to the capacitance of the piezoelectric element is the discharging time or a function of the discharging time, if the difference between the third voltage and the second voltage and the current discharging the piezoelectric element are kept essentially constant (with respect to the time instances the values related to the capacitance is calculated for or with respect to different piezoelectric elements). In a further embodiment of the invention the value related to the capacitance of the piezoelectric element is the current discharging the piezoelectric element or a function of the current if the difference between the third voltage and the second voltage and the discharging time are kept essentially constant (with respect to the time instances the values related to the capacitance is calculated for or with respect to different piezoelectric elements).
In a further preferred embodiment of the invention the control unit determines the value related to the capacitance of the piezoelectric element based upon the quotient of the third voltage and the charge the piezoelectric element is carrying, based upon the quotient of the charge the piezoelectric element is carrying and the third voltage, based upon the quotient of the difference between the second voltage and the third voltage and the product of the discharging time and the current discharging the piezoelectric element, or based upon the quotient of the product of the discharging time and the current discharging the piezoelectric element and the difference between the second voltage and the third voltage. If one of the quantities charging time, current charging the piezoelectric element or difference between second voltage and the first voltage is kept essentially constant (with respect to the time instances the values related to the capacitance is calculated for or with respect to different piezoelectric elements), it can be replaced by one or another constant value.
In a further embodiment of the invention the third voltage equals the first voltage.
In a further preferred embodiment of the invention the control unit is able to determine a possible fault of the piezoelectric element or electric circuitry driving the piezoelectric element based upon a calculated value of the capacitance of the piezoelectric element and a value related to the capacitance of the piezoelectric element at a former stage, in particular based upon a comparison of the value related to the capacitance of the piezoelectric element and a former value related to the capacitance of the piezoelectric element.
In a further preferred embodiment of the invention the fuel injection system according to one of the foregoing claims wherein the fuel injection system comprises a switch (11, 21, 31, 41, 51 or 61) for discharging the piezoelectric element, wherein the control unit is able to determine a possible short cut of the switch (11, 21, 31, 41, 51 or 61) based upon the value related to the capacitance of the piezoelectric element.